In order to install RapidWeaver on your other machine, please download the latest version of RapidWeaver 6 and install it on your new machine.
In order to move the third-party add ons, you will have to follow these next steps:
• Open RapidWeaver 6
• While holding the ⌥-key down, head over to the menu and select [File > Reveal Addons Folder] (please refer to this screenshot)
• Quit RapidWeaver
• You will have to copy the contents of this previous folder on to your secondary computer, and add them to the same folder on your other Mac.
Finally, when working across multiple computers, it is very important that you do not store your .rw6 RapidWeaver Project File (or .rwsw for RapidWeaver 5) on Dropbox (or similar cloud synching solutions) The best way to work across multiple computers would be to save your project locally, then right-click on the project file and select [Compress…]. This will add the project inside a .zip archive. You can them move the .zip archive via USB or Dropbox. Please make sure to always save your projects locally.
Let me know if there is anything else I can do to help you.